The Irma synthetic signature model was one of the first high resolution infrared (IR) target and background signature models to be developed for tactical weapons application. Originally developed in 1980 by the Munitions Directorate of the Air Force Research Laboratory, the Irma model was used exclusively to generate IR scenes for smart weapons research and development. In 1988, a number of significant upgrades to Irma were initiated including the addition of a laser channel. This two channel version, Irma 3.0, was released to the user community in 1990. In 1992, an improved scene generator was incorporated into the Irma model which supported correlated frame-to-frame imagery. This and other improvements were released in Irma 2.2. Irma 3.2, a passive IR/millimeter wave (MMW) code, was completed in 1994. This served as the cornerstone for the development of the co- registered active/passive IR/MMW model, Irma 4.0. Currently, upgrades are underway to include a near IR (NIR)/visible channel; a facet editor; utilities to support image viewing and scaling; and additional target/data files. The Irma 4.1 software development effort is nearly completion. The purpose of this paper is to illustrate the results of the development. Planned upgrades for Irma 5.0 will be provided as well. Irma is being developed to facilitate multi-sensor research and development. It is currently being used to support a number of civilian and military applications. The current Irma user base includes over 100 agencies within the Air Force, Army, Navy, DARPA, NASA, Department of Transportation, academia, and industry.